143 Reorder List

Reorder List

You are given the head of a singly linked-list. The list can be represented as:

L0 → L1 → … → Ln - 1 → Ln

Reorder the list to be on the following form:

L0 → Ln → L1 → Ln - 1 → L2 → Ln - 2 → …

You may not modify the values in the list’s nodes. Only nodes themselves may be changed.

 

Example 1:

**Input:** head = [1,2,3,4]
**Output:** [1,4,2,3]

Example 2:

**Input:** head = [1,2,3,4,5]
**Output:** [1,5,2,4,3]

 

Constraints:

The number of nodes in the list is in the range [1, 5 * 104].
1 <= Node.val <= 1000

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:


    temp = None


    def reorderList(self, head: Optional[ListNode]) -> None:
        """
        Do not return anything, modify head in-place instead.
        """
        if head.next == None:
            return

        self.temp = head
        result = head
        self.Traverse( head, None)
        head = result
    
    def Traverse(self, head : ListNode, prev : ListNode) :
        if head == None:
            return
        
        self.Traverse( head.next, head)
        

        if head.next == None and prev != self.temp and self.temp != None and self.temp.next != None:
            print(head, prev)
            temp1 = self.temp.next
            self.temp.next = head
            head.next = temp1
            self.temp = temp1
            if prev != None:
                prev.next = None